The present invention relates to an acceleration switch, and more particularly, to an acceleration switch including a movable electrode that contacts a fixed electrode when subjected to acceleration that exceeds a predetermined value.
Many automobiles are presently equipped with air bag systems. A typical air bag system includes an air bag, an ignitor, and an electronic control unit (ECU). The ECU includes an acceleration sensor, which detects a sudden change in acceleration upon collision of the vehicle. The employment of a mechanical acceleration switch (i.e., safing sensor) has been proposed as such acceleration sensor.
FIGS. 1A to 1C schematically show the structure of a prior art acceleration switch 51. The acceleration switch 51 includes a silicon chip 52 and a substrate 53, which are connected to each other. As shown in FIG. 1B, the silicon chip 52 has a length of L1 and a width of W1. Further, the silicon chip 52 has a hollow portion 52a in which a block-like inertia weight 54 is arranged.
A beam 55 is provided on each long side of the inertia weight 54 and extends from a position offset from the middle of the long side. The beams 55 connect the inertia weight 54 and the silicon chip 52. The inertia weight 54 pivots about the beams 55 at a position offset from the center of gravity of the inertia weight 54. Referring to FIG. 1B, the beams 55 each have a length of T2. Two movable electrodes 56, 57 are arranged on the lower side of the inertia weight 54, as viewed in FIG. 1A. The movable electrodes 56, 57 are located close to each other at the middle of the weight end that is farther from the beams 55.
A hollow portion 53a is defined in the upper surface of the substrate 53. A fixed electrode 58 is formed in the hollow portion 53a at a position corresponding to the movable electrodes 56, 57. The movable electrodes 56, 57 are normally separated from the fixed electrode 58.
When the acceleration switch 51 is subjected to acceleration, inertial force pivots the inertia weight 54 about the axis of the beams 55 in a downward direction (the direction indicated by arrow G in FIG. 1A). When the acceleration becomes greater than or equal to a predetermined value, the inertia weight 54 pivots in a direction indicated by arrow F in FIG. 1A, and the movable electrodes 56, 57 contact the fixed electrode 58. When the value of acceleration is small, the movable electrodes 56, 57 do not contact the fixed electrode 58. Accordingly, the acceleration switch 51 is actuated only when the acceleration becomes greater than or equal to a predetermined value.
To actuate the acceleration switch 51 at a relatively small acceleration, the beams 55 may be thinned or elongated. However, the formation of thinner beams 55 has physical limitations and is thus not effective. Further, the formation of longer beams 55 limits miniaturization. For example, when the length of the beams 55 is changed to T3 (T2 less than T3) as shown in FIG. 2, the width W2 of the silicon chip 52 is increased by 2xc3x97T3xe2x88x922xc3x97T2 in comparison to when the width of the silicon chip 52 is W1 (FIG. 1B). This increases the area of the silicon chip 52 and enlarges the acceleration switch 51.
It is an object of the present invention to provide an acceleration switch having improved detection sensitivity while avoiding enlargement.
To achieve the above object, the present invention provides an acceleration switch including a switch body, a fixed electrode arranged in the switch body, and a movable weight arranged in the switch body. The movable weight is displaced when subjected to acceleration. The movable weight includes a movable electrode that contacts the fixed electrode when the movable weight is displaced, a recess formed in a side of the movable weight, and a beam connecting the movable weight and the switch body. The beam pivotally supports the movable weight and extends into the recess.
Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.